1. Field of the Invention
This invention relates to a rotary tool holder, and more particularly to a tool holder for limiting a torque to be transmitted by a spindle of a machine tool or the like to a tool, such as a drill, tap or reamer, within a range in which such a tool does not suffer from any damage or break.
2. Description of Prior Arts
In order to protect a tool from an unusually large torque due to an increased rotational resistance imposed by a workpiece, which torque would be a cause of damaging the tool, a variety of holders incorporated a different safety mechanism have hitherto been proposed.
One of prior art holders, which is disclosed in Japanese Laid-open Utility Model Publication No. Sho 51-113883, has been such that a rotary driving member to be coupled to a spindle and a driven member receiving therein a tool are connected to each other by a torque transmitting means having two or more balls; and such balls are usually maintained in engagement with grooves provided in the driven member at the transmission of a usual torque, but in the event of an unusually large torque being transmitted to the driven member, such balls disengage from these grooves to interrupt transmission of such an unusually large torque to the driven member. With such a holder, retransmission of a torque is achieved by the subsequent, angular idling of the torque transmitting means in the same direction as before, for reengagement of such balls with these grooves. However, retransmission of torque, stated otherwise, reengagement of such balls with these grooves, is accompanied by strong vibration. Such vibration occurs during the rotation in the normal direction, of the tool, stated otherwise, during the tapping, drilling or reaming operation for workpiece, thus resulting in the lowered precision in operation. Particularly such vibration that occurs during a tapping operation greatly impairs the precision in tapping.
It is generally known of the operation of a tap that, upon returning to its start position through its rotation in the reverse direction, the tap receives resistance due to chips of a work involved in the tap, having a likelihood of failing to be returned to its start position. To avoid this, a torque is needed, which is larger than that exerted on the tool at the rotation in the normal direction. For this purpose, there is desired a safety mechanism for providing a difference between a torque to be transmitted to the tool at the rotation in the normal direction of the tool and that at the rotation in the reverse direction thereof.
It is accordingly a primary object of the present invention to provide a rotary tool holder, wherein a tool is maintained independent of vibration even in the case of retransmission of a torque to the tool in operation, following the interruption of transmission of such torque.
It is another object of the present invention to provide a rotary tool holder, wherein in the event that transmission of a torque to a tool is interrupted during rotation in the normal direction, of the tool, then retransmission of a torque to the tool in operation cannot be realized unless the tool is rotated in the reverse direction.
It is a further object of the present invention to provide a rotary tool holder, wherein a torque to be transmitted to the tool at the rotation in the reverse direction is larger than that to be transmitted to the tool at the rotation in the normal direction.
It is a still further object of the present invention to provide a rotary tool holder, wherein a value of torque to be transmitted to a tool is variable, irrespective of a direction of rotation of the tool.
To attain these objects, there is provided according to the present invention a rotary tool holder which comprises; a rotary driving member adapted to be coupled to a spindle of a machine tool; a cylindrical driven member having an inner bore for receiving therein a tool, such as a drill, tap or reamer, a plurality of peripheral dents or shallow grooves spaced apart from one another in the circumferential direction of the driven member, and recesses adjoining to these dents; a sleeve member for transmitting a torque from the rotary driving member to the driven member and having first rolling members engageable with the dents, and a plurality of slots in which respective rolling members are adapted to be loosely fitted, the slots being arranged so as to displace the rolling members in the axial direction of the sleeve member, when rotation of the sleeve member is reversed; an outer sleeve member retaining the sleeve member on the driven member; a cup-shaped member slidably disposed between the sleeve member and the driven member and having a wall inclined radially inwardly thereof; resilient means for urging the cup-shaped member against these rolling members; and means disposed in the recesses in the driven member for releasably impeding a sliding movement of the cup-shaped member toward the rolling members. By such arrangements, transmission of a torque to a tool is interrupted during the rotation in the normal direction of the sleeve member, whereas the sleeve member continues idling. The idling of the sleeve member is terminated by reversing the rotation of the sleeve member; the sleeve member is then rotated in the normal direction so as to transmit a torque to a tool for its operation; and at the return or withdrawal of the tool from a workpiece through the rotation in the reverse direction, a torque larger than that at the rotation in the normal direction is transmitted to the tool, so as to prevent the tool from being damaged as well as to ensure the withdrawal of the tool from the workpiece.